1. Field of the Invention
The present invention relates to an optical information medium, on which can be recorded information which is optically reproducible, and a recording method for recording signals that are optically readable, and in particular relates to the optical information medium and the recording method, for realizing a recording/reproducing system suitable for a high density recording medium on which the information can be recorded and reproduced with red color laser beam of a short wavelength of from 630 nm to 670 nm.
2. Description of Related Art
With the recent advances in development and application of a short wavelength laser, standardization has proceeded on a standard for DVDs (Digital Versatile Discs) which enables recording and reproducing at a high density, as well as the practical application thereof. The DVD has a data area at least provided on one surface of the substrate thereof, so as to form so-called pits as a means for information thereon, and a reflection layer made of metal film which is formed on that data recording area.
For the DVDs, a different standard for high density is determined, compared to the standard for the optical information medium being the most general or popular now, such as a CD (Compact Disc). For example, with the optical pickup, it is determined to use the red color laser of short wavelength from 630 nm to 670 nm, and to use an objective lens of high numerical aperture (NA), such as of 0.6 in NA, etc. Accompanying to this, for dealing with a disc tilt, the thickness thereof is determined 0.6 mm, to be about half (xc2xd) of that of the CD. However, for maintaining compatibility or interchangeability in sizes between the CD having thickness of 1.2 mm, the disc of DVDs applies such structure that two pieces of discs are aligned and bonded together. According to the DVD standard, it is standardized that one layer of the disc has recording capacity of 4.7 GB at the maximum, in average thereof, i.e., video and audio information for 133 minutes long.
In the standardization of the DVD standard, in the similar manner of the relationship between the CD-ROM and CD-R or CD-RW, at the beginning were assumed both an optical information medium for use of reproduction exclusively (for reproduction only) and an optical information medium being recordable. However, the standardization at the beginning was made only on a DVD-Video for video and on a DVD for reproduction only, such as DVD-ROM which is targeted for recording computer data, but the standardization on the recordable DVD was postponed. Under such a situation is desired development of an optical information medium, on which recording is possible with high density at the maximum recording capacity of 4.7 GB on one layer of the disc.
With such a DVD, i.e., the high density recording medium as mentioned in the above, the recording is performed with a high density, tracking pitch of 0.74 xcexcm and minimal pit size of 0.4 xcexcm (0.44 xcexcm in case of double layer recording), comparing to a tracking pitch of 1.6 xcexcm and minimal pit size of 0.83 xcexcm in the CD. A means for enabling the reproduction with the pits of such a high density is the use of red color laser of a short wavelength from 630 nm to 670 nm and the use of objective lens of a high numerical aperture (NA), such as of 0.6 in NA, as mentioned in the above.
On the surface of the substrate of these recordable DVDs, a groove of a pitch of 0.74 xcexcm as mentioned above is formed for tracking, and on the surface of the substrate having that groove is provided a recording layer including an organic dye. Further on the organic dye is provided a reflection layer, such as of Au, Ag or Al, etc. For recording signals on that recordable DVD, there is a necessity to radiate a recording laser beam upon the recording layer mentioned above, so as to form the pits of various sizes, including the minimum size of 0.4 xcexcm. For that purpose, the red color laser of a short wavelength from 630 nm to 670 nm is used, and the beam spot thereof must be focused onto a very narrow region to perform the recording.
A first object, in accordance with the present invention, for solving the drawbacks in the technology for performing recording on the optical information medium corresponding to the high density recording, is to provide a recording method for an optical information medium, able to record optically readable signals at a high density. A second object, in accordance with the present invention, is to provide an optical information medium which enables stable recording of the signals, thereby easily obtaining a reproduction signal satisfying the standard determined in relation to the high density information medium, such as the DVD standard mentioned above.
As mentioned previously, in a DVD player for reproducing from a high density recording medium, such as a DVD, which is standardized at the present time, the numerical aperture (NA) of the objective lens of the optical pickup is 0.6, for radiating reproduction laser beam. The larger the aperture of the objective lens in the optical pickup, in other words, the larger the NA of the objective lens, the smaller the beam spot can be focused upon the optical information medium, thereby enabling reproduction at a high density.
In the similar manner, also when recording onto the optical information medium, the larger the NA of the objective lens for radiating the recording laser beam, the smaller the beam spot of the recording laser beam that can be radiated upon the optical information medium, thereby enabling recording at a high density.
However, when the NA of the objective lens is large, the focus point is likely to shift due to the inclination of the disc to an optical axis of the objective lens caused by the curvature of the discs, therefore, there may easily occur a problem, such as deterioration in the deviation of pit-and-land length, accompanying with the worth in balance between or among the pits of various length, including long and short ones which are formed through the recording operation. Accordingly, in accordance with the present invention, first of all, the NA of the objective lens in the optical pickup when recording is larger than that when reproducing. Second, for enabling stable recording of signals, even under the condition where the objective lens (o) has a large NA, an index xe2x88x9d for the film thickness of a land/groove of said recording layer is expressed by 1xe2x88x92Dr/Ds when assuming that depth of said groove is Ds and depth of said recording layer at the position of said groove is Dr, and the width of the groove 3 are selected, appropriately.
Namely, according to the present invention, there is provided a recording method for an optical information medium, comprising: a transparent substrate 1 for allowing a recording laser beam to penetrate therethrough; a recording layer 12 formed on said transparent substrate 1; and a reflective layer 13 formed on said recording layer 12 for reflecting the reproducing laser beam thereupon, wherein, when recording optically readable signals by means of the recording laser beam incident from said transparent substrate 1, the NA of an objective lens (o) of an optical pickup for irradiating the recording laser beam on said recording layer 12 is selected to be approximately 0.65, while the NA of an objective lens of the optical pickup for irradiating the reproducing laser beam on said reflecting layer 13 is selected to be approximately 0.6.
In more details, the NA of the objective lens (o) of the optical pickup for irradiating the recording laser beam on said recording layer 12 is determined to be 0.65xc2x10.02, and the NA of the objective lens of the optical pickup for irradiating the reproducing laser beam is determined to be 0.60xc2x10.02. In this instance, the wavelength of the recording laser beam lies from 630 nm to 670 nm, in the same region of the wavelength of the reproducing laser beam.
When this recording method for the optical information medium, since the NA of the objective lens (o) of the optical pickup is selected to be approximately 0.65 and larger than the NA of 0.6 for the reproducing laser beam, the beam spot can be focused smaller when recording. Therefore, the recording power can be focused into a spot smaller in beam diameter than in the conventional art.
On the other hand, as mentioned above, when the NA of the objective lens is large, the focus point is likely to shift due to the inclination of the disc to an optical axis of the objective lens caused by the tilt of the discs, therefore, there may easily occur a problem, such as a deterioration in the deviation of pit-and-land length, accompanying with the worth in balance between or among the pits of various lengths, which are formed through the recording.
Then, according to the present invention, an inclination angle at both sides of said groove with respect to the recording laser beam incident surface, which is the main surface of said transparent substrate, is selected from 55xc2x0 to 75xc2x0. With this, even with the beam which is focused by the objective lens (o) with a large NA, because of accurate tracking and increase in contrast due to light interference at the pit edge portion when recording, the modulation amplitude of all the long and short pits are optimized.
For enabling accurate tracking and recording corresponding to the DVD standard under the condition where the NA of the objective lens (o) is 0.65xc2x10.02, the inclination angle xcex8 must be equal to or greater than 55xc2x0, and the larger the better, under the condition that the transparent substrate 1 has a thickness of 0.6xc2x10.02 mm and a spiral groove 3 for tracking at a pitch of 0.74xc2x10.01 xcexcm, formed on the surface on which the recording layer 12 is formed. However, if the inclination angle of the groove 3 exceeds 75xc2x0, the transparent substrate 1 is difficult to remove from a mold when produced by a method of injection molding, thereby bringing about a remarkable decrease in quality of the product and deterioration in productivity thereof. Therefore, the inclination angle xcex8 must be determined not to exceed 75xc2x0.
Further, when assuming that the depth of the groove 3 on the transparent substrate 1 is Ds and the depth of the recording layer at the position of said groove is Dr, the index xe2x88x9d for the film thickness of the land/groove, expressed by 1xe2x88x92Dr/Ds, is selected from 0.2 to 0.4. This index xe2x88x9d=1xe2x88x92Dr/Ds for the film thickness of the land/groove indicates the degree of leveling of the recording layer 12 when the recording layer 12 is formed by applying the organic dye on the transparent substrate having the groove 3 for tracking, by means of the spin coating method. The value of this film thickness index xe2x88x9d indicates that the ratio between the depth of the groove on the surface of the recording layer 12 and the depth of the groove 3 on the surface of the transparent substrate 1 comes to be larger as the result of proceeding or advancing of the leveling on the recording layer.
This film thickness index xe2x88x9d is preferably small from the view point of the modulation amplitude which is obtained when being reproduced after recording. For obtaining the higher modulation amplitude of the DVD standard, under the condition that the NA of the objective lens (o) is 0.65xc2x10.02 when recording and the NA of the objective lens (o) is 0.60xc2x10.02 when reproducing, the index xe2x88x9d must be equal to or less than 0.4 (xe2x88x9dxe2x89xa60.4).
On the other hand, if this index xe2x88x9d=1xe2x88x92Dr/Ds is too small, as a result of a small degree in leveling, the concave/convex of the reflective layer 13 comes to be large, thereby reducing the reflectivity. For obtaining high reflectivity, with the reflectivity being equal to or greater than 45% being determined in the DVD standard, the index xe2x88x9d is selected to be equal or greater than 0.2 (xe2x88x9d greater than 0.2).
Further, a full width at half maximum (FWHM) of the groove 3 on the transparent substrate 1 is selected from one-third (⅓) to a half (xc2xd) of the pitch p of the groove 3. The FWHM of the groove 3 means the width of the groove at the position where the depth Ds is a half (xc2xd) of the depth Ds of the groove 3 which is formed on the transparent substrate 1. When the NA of the objective lens (o) is set to be 0.65xc2x10.02 when recording, although previously mentioning that the beam spot of the recording laser beam can be made smaller, however, if the FWHM of the groove 3 is widened to exceed. a half (xc2xd) of the pitch p (0.74 xcexcm) of the groove 3, no effect can be obtained by focusing the recording laser beam to the beam spot. On the other hand, if the FWHM of the groove 3 is narrowed so as not to reach to one-third (⅓) of the groove 3, sufficient modulation amplitude cannot be obtained when reproducing by selecting the NA of the objective lens (o) to be approximately 0.60xc2x10.02.
According to the present invention, when recording signals onto the optical information medium to meet the high density recording, it is possible to perform recording while focusing the recording laser beam into a small region or spot, by means of the optical pickup having the objective lens (o) of the NA 0.65xc2x10.02 larger than the NA 0.60xc2x10.02 of the objective lens for reproducing, thereby enabling recording at a high density. Moreover, the drawbacks which are likely to occur accompanying the enlargement in the NA of the objective lens (o) in the optical pickup can be solved by selecting the inclination angle of both sides of the groove for tracking on the transparent substrate of the optical information medium, the ratio between the width and pitch of the groove, or the index for film thickness of land/groove of the recording layer, appropriately. With this, it is possible to obtain reproduced signals which satisfy the requirements determined for a high density optical information medium, such as the DVD standard, with ease.